This invention relates to compression-type loudspeakers driven by an annular diaphragm, and more particularly to an improved acoustical transformer or phasing plug for use in such loudspeakers.
In compression-type loudspeakers, an acoustical transformer, commonly known as a "phasing plug", is used to improve the impedance match between the output of the speaker diaphragm and the throat of the horn. The improved impedance match allows more acoustic power to be transferred from the diaphragm, particularly at low frequencies. Furthermore, the phase plug reduces the apparent size of the annular diaphragm, thus improving high frequency response and dispersion. In most applications, the throat diameter at the horn is small compared to the diameter of the annular diaphragm.
In the prior art, acoustical transformers or phase plugs for use with compression-type loudspeakers driven by an annular or ring diaphragm have consisted of a plug having an annular slot located next to, and concentric with, the annular ring diaphragm. The phase plug contained an annular, axially symmetric passageway connecting the annular slot to the mouth of the horn. The annular passageway typically expanded in cross section from the diaphragm to the throat so as to nearly cover the entire throat of the horn. However, the phasing plug utilizing an annular slot adjacent to the diaphragm exhibits poor dispersion characteristics at higher frequencies because the apparent size of the source is large compared to the wavelength.
Other prior art, such as that described in U.S. Pat. No. 4,050,541, utilizes a series of radial slots adjacent to a domeshaped diaphragm, but that invention does not teach the use of channels with an annular diaphragm.